Induction heat treatment is known to be an effective method of case hardening certain nodular iron alloys. However, the use of induction hardening for nodular iron components has been limited in cases where the portions of the component requiring heat treatment cannot be uniformly heated with an induction coil of conventional design, such as a cylindrical coil, perhaps due in part to the complex microstructural balances required as well as the difficulty associated with designing induction coils to provide the necessary heating of irregularly shaped surfaces. While some solutions have been proposed to facilitate the use of induction hardening with articles having irregular surfaces, such as the use of different coils and different induction frequencies to treat different portions of the surface, or the use of coil designs that are adapted to the contour of the irregularities in order to provide more uniform inductive coupling, induction hardening has not been used for various types of components, such as those described below.
Nodular iron is widely used to cast various components, including various housings used in automotive vehicles. For example, nodular iron is commonly used to make a variety of differential housings, which are used to house the differential gears for rear wheel drive vehicles. Because it is widely used in this application, nodular iron differential housings are manufactured in relatively high volumes. Nodular iron differential housings typically have bell-shaped outer surfaces extending along a longitudinal axis, while the internal housing portion is frequently spherical or partially spherical and adapted to house a differential gear set. The open end of the bell-shaped surface frequently incorporates a flange that is adapted to receive a ring gear, which in turn transitions to a lower hub at the lower end of the housing that is adapted to receive an axle shaft. The closed end of the bell-shaped surface also frequently transitions to an upper hub that is adapted to receive an axle shaft. All of these features are typically cast at once as what may be described as a single wall or sidewall. The bell-shaped surface also frequently includes opposed front and rear openings which extend through the sidewall of the housing into the internal housing portion. These openings may be cast into the housing, or as is frequently the case, machined while the castings are still warm from the casting process.
In addition to housing the differential gear set, a primary function of the differential housing is to transmit torque from the drive train to the drive wheels through the differential gears. As a result, differential housings typically have rigorous metallurgical and mechanical requirements. As the torque capacity requirements for a particular differential housing design are increased, the mechanical and metallurgical requirements for that housing design are also increased. As these requirements are increased, Applicants have observed that they occasionally exceed the torque handling capacity of the housing, leading to fracture of the housing in the area having the lowest torque handling capacity. This is typically in a region adjacent the front and rear openings as described above, and more specifically between the openings and the upper hub.
Increased capacity can be accomplished by increasing the size of the housing, but it is frequently desirable to maintain or reduce the size of the housing while also increasing its torque handling capacity.
Therefore, it is desirable to develop a method of heat treatment that addresses the limitations mentioned above and that provides a method for surface or case hardening parts having an irregular surface, such as a differential housing, in a specific region of the surface, whereby the torque handling capacity of a given differential housing design may be increased while maintaining the size of the housing, or potentially enabling a reduction in the size of the housing.